LAUSANNE, Switzerland — Nuclear negotiations between the United States and Iran entered a critical phase on Thursday with U.S. Secretary of State John Kerry meeting his Iranian counterpart less than a week away from a deadline to secure the outline of a deal.

With the clock ticking, Kerry and Iranian Foreign Minister Mohammad Javad Zarif and their teams huddled Thursday in the Swiss resort town of Lausanne on Lake Geneva trying to overcome still significant gaps after nearly two years of negotiations between Iran and the five permanent members of the U.N. Security Council and Germany. The top diplomats from Britain, China, France, Germany and Russia are expected to join the talks if the U.S. and Iran are close to an agreement.

U.S. officials say the March 31 deadline is achievable but remains uncertain. En route to Switzerland with Kerry on Wednesday, one official said the American side can see a path to get to an agreement by the end of March as the last round of talks produced more progress than many previous rounds. The official was not authorized to discuss the talks by name and spoke on condition of anonymity.

The Iranian side was more upbeat. Ali Akbar Salehi, Tehran's top nuclear official, told Iran's IRNA news agency that the talks have already reached a "common understanding" on technical issues. Salehi, who also is at the talks, added he was optimistic that a comprehensive deal also was within reach.

The pressure is high. The seven nations have set themselves a March 31 deadline for the outline of a final accord they hope to seal by the end of June. Both President Barack Obama and Iran's Ayatollah Ali Khamenei have spoken against what would be a third extension of the talks.

And, looming over this round of talks are the crises in Yemen, where U.S. allies, including Saudi Arabia, have launched air strikes against Iranian-backed Shiite rebels that toppled the government, and Iraq, where the U.S. is now providing air support to the Iraqi government's Iranian-backed offensive to retake the city of Tikrit from Islamic State group militants.

At the opening session of Thursday's talks neither Kerry nor Zarif responded to reporters' questions about whether the situation in Yemen would be discussed.

But the two discussed the situation in the Arabian country at a private one-on-one meeting, State Department spokesman Jeff Rathke said. He declined to provide details of the discussion other than to say it was brief. Kerry also spoke by telephone Thursday with Arab foreign ministers and "commended" them on taking military action, Rathke said.

Opponents of a nuclear deal, among them wary American allies in the Middle East and hardliners in Iran and in Congress, stand ready to complicate the process if negotiators cannot reach a breakthrough in the next six days. American lawmakers have threatened new sanctions on Iran as well as the establishment of a process which would allow them to vote down any final accord.

The United States and its partners are trying to get Iran to cut the number of centrifuges it uses to enrich uranium, material that can be used in warheads, and agree to other restrictions on what the Islamic Republic insists is a peaceful nuclear program.

Iranian President Hassan Rouhani tweeted that he had written to the leaders of the six world powers involved in the negotiation, explaining his country's position. French President Francois Hollande's office also said Hollande and Rouhani spoke by telephone and Hollande urged progress toward a "lasting, robust and verifiable" deal.

National Security Council spokeswoman Bernadette Meehan said the letter from Rouhani to Obama was passed on to the negotiating team in Switzerland.

Speaking Wednesday morning to U.S. ambassadors in Washington, Kerry assailed opponents of a deal.

"What happens if, as our critics propose, we just walk away from a plan that the rest of the world were to deem to be reasonable?" Kerry asked. "Well, the talks would collapse. Iran would have the ability to go right back spinning its centrifuges and enriching to the degree they want... And the sanctions will not hold."

Kerry said the whole point of years of U.S. sanctions was to get Iran to agree to limits on its nuclear program. He said it was the Obama administration's job to "provide an agreement that is as good as we said it will be; that will get the job done; that shuts off the four pathways to a nuclear weapon."

Associated Press writers Bradley Klapper in Washington, Ali Akbar Dareini in Tehran, Iran; George Jahn in Vienna and Sylvie Corbet in Paris contributed to this report.